In the art of sheet metal work, it is frequently necessary to provide a sequence of precision bends according to a predetermined bend angle and position. A preferred manner of bending sheet metal provides the use of a two-piece metal die having complementary surfaces, wherein the sheet metal to be bent is inserted between the two die pieces which are driven together. The sheet metal bend is formed according to the shape of the interface surface between the two die pieces and the extent to which the two die pieces are driven together. While it is possible to configure the die pieces in a variety of bending shapes, for the purpose of the discussion here, the cross-section of the die piece interace will be assumed to be a V-shape: that is, the lower piece to be a V-groove (die) and the upper piece to be a complement of the V-groove (punch) fitting within the lower piece and the sheet metal to be placed therebetween.
The apparatus used to move the die or punch can include pneumatic, hydraulic or mechanical apparatus. The pneumatic and hydraulic devices are pistons actuated by valves in the pneumatic or fluid lines. The mechanical apparatus typically include a two-part mechanical actuating device attached to the upper punch piece, having an adjustable extension where the extension piece may be moved over several inches as desired, and a cyclical eccentric which is attached to the upper die piece through the extension piece. The eccentric rotates through one complete cycle having a predetermined and fixed motion sequence. The sum of the extension position, the eccentric motion and the thickness of the sheet metal to be bent must be exceed the distance to the lower die surface (usually fixed) to prevent the risk of fracturing the die pieces.
Furthermore it is generally known within the art of metal bending to form a smaller bend (at an angle greater than the apex angle of the V-groove) by limiting the downward extension of the die to less than the total distance necessary to achieve complete contact of the two die pieces on both sides of the metal to be bent. This technique is known as air-bending.
In the operation of the above-described metal bending apparatus, or press brake, there exists a need for great accuracy in the control of the die motion. This means the allowable error is typically in the neighborhood of several thousandths of an inch of displacement. Furthermore, in mass production operations, this precision control must be maintained and repeated throughout the sequence of bend operations to be performed.
Moreover, there exists a great number of manually or partially automated press brake machinery now used by the sheet metal industry. To implement the above-described functions with the necessary precision would require substantial if not complete reworking or replacement of the existing machinery. It is therefore necessary to seek out an alternative approach which would allow for acceptable retrofitting of these machines to provide the necessary automatic machine control.